A study in the upper Illinois River Basin defined relations between changes in wastewater-treatment practices and changes in stream-water quality on the basis of available information. These relations were examined for five large wastewater-treatment plants in the Chicago area, Illinois. At the three largest treatment plants, two major changes in wastewater-treatment practices were identified--the cessation of chlorination and the implementation of Chicago's Tunnel and Reservoir Plan (TARP). Other changes, such as improved aeration and expansion of the facilities, also were made at some of the treatment plants. At the Calumet Water Reclamation Plant, median densities of fecal coliform bacteria in the effluent increased from 3,100 to 1,200,000 colonies per 100 milliliters after the cessation of chlorination. Median densities at the nearest downstream monitoring site increased from 9,500 to 250,000 colonies per 100 milliliters. Similar changes in bacteria densities were indicated for other treatment plants and stream-monitoring sites, but increases in densities of fecal coliform bacteria were not indicated at distances greater than 7 miles downstream. Substantial changes in effluent and stream-water quality, primarily improvements, were identified after the implemen- of TARP and improvements in aeration. Decreases in some of the largest concentrations of ammonia were particularly notable and were likely results of the cape and treatment of combined sewer overflows by TARP. Improvements in water quality were commonly related to climatic season, with greater changes taking place during warm periods. Substantial decreases in concentrations were identified for many constituents, including oxygen demand, ammonia, bacteria, and cyanide. The water-quality data available for this study were considered to be more accurate and were more comprehensive than data from most other monitoring programs. The results of this study, however, identified some needed enhancements to increase the usefulness of the data for additional purposes and analyses.